Oh, didn't saw that I will try that later and repost here.
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So, you are proposing this way of alternative doing the NAT (I've applied some queues for testing):

I'm still wandering why queueing works for downloads (direction: <data> -> $ext_if -> $int_if -> <network>) while the rule is read in the opposite direction (from <network> to <data>). Can you put some light on that? What rule should I use if I want to limit upload speed?